With recent decreases in costs of storage media such as DRAMs and HDDs, it is becoming possible to inexpensively store a large amount of image data in an image forming apparatus. This makes it possible to add a storage (filing) function to the conventional image forming apparatuses having only functions of printing out data onto sheets of paper and outputting image data read from originals. While the convenience improves because a large amount of images can be stored in an image forming apparatus having a storage device, a new security problem arises. That is, the possibility that image data on the storage device is tampered, erased, or illegally output by an unauthorized third party is pointed out. To prevent this problem, the conventional image forming apparatuses protect unauthorized access to image data by personal authentication using a password or the like (e.g., Japanese Patent Laid-Open No. 2002-218220). Also, with the recent progress of time business, a technique which certifies the validity of data by communication with a time stamp authority or certificate authority on the Internet has been developed and put into practical use. For example, a time stamp method complying with RFC3161 is beginning to be used.
When digital data is to be protected by certifying the validity of data by communication with a certificate authority or time stamp authority on the Internet, a user sometimes wants to output the contents of data in accordance with the degree of urgency even if:
1) the data to be output is not authorized by the certificate authority because, e.g., the data is tampered.
2) the system of the authentication server is not operating for some reason, or
3) the user cannot communicate with the authentication server for some reason such as a network failure.
Under any of conditions 1) to 3) above, however, the output result of image data may be abused because the data is tampered, or data whose validity is not certified by the certificate authority or time stamp authority may be carelessly distributed.